Idiopathic (primary) pulmonary arterial hypertension (IPAH), a subgroup of plexogenic pulmonary arterial hypertension (PAH), is a rare disorder associated with severe morbidity and high mortality rates. There are no routine screening tests or validated markers of disease activity in IPAH, or the broader group of PAH. Therefore, patients usually present at advanced stages of disease. The pathogenesis of IPAH and other forms of PAH remain unclear. Current thinking focuses on a two-hit hypothesis: 1) genetic susceptibility, and 2) a triggering stimulus that initiates pulmonary vascular injury, resulting in endothelial cell dysfunction. Endothelial cells are normally shed into the circulation and are a valuable source of clinical material for studying diseases characterized by endothelial cell dysfunction. Unfortunately, no clear methodology exists for isolating clinically relevant numbers of circulating endothelial cells (CECs). In the bench phase of the project we are using flow cytometry to develop a methodology for isolating clinically relevant numbers of viable CECs from healthy volunteers and patients with PAH. We hypothesize that CECs can be used to define a subset of differentially regulated biomarkers in IPAH and other forms of PAH that may lead to earlier diagnosis and better methods for measuring responses to therapy. We also hope to identify novel targets for future therapeutic interventions. In the clinical phase of the project, we recruited healthy volunteers and patients with IPAH and other forms of PAH (vascular injury induced pulmonary hypertension). Peripheral blood specimens were obtained for microarrays. A subset of subjects underwent right heart catheterization to assess pulmonary pressures and to obtain pulmonary blood specimens. We started actively enrolling into the protocol in June 2006. We have enrolled 31 individuals to date. Our preliminary data suggests that there was no trend towards CEC enrichment in pulmonary vein blood compared to peripheral blood (PB) for both the healthy volunteers (4.4 CEC/ml vs. 4.8 CEC/ml) and the PAH patients (2.4 CEC/ml vs. 3.0 CEC/ml). There was a trend towards CEC enrichment in pulmonary artery blood compared to PB for both the healthy volunteers (13.8 CEC/ml vs. 4.8 CEC/ml) and the PAH patients (3.3 CEC/ml vs. 3.0 CEC/ml). The protocol is closed to further enrollment, but remains open for data analysis. In particular, total RNA for microarrays will be prepared from peripheral blood mononuclear cells which will then be studied in depth using high density oligonucleotide microarrays to more fully characterize their transcriptome.